Thermoacoustic devices have been used as heat engines and heat pumps. As shown in FIG. 1, one mechanism for manipulating thermoacoustic waves is a conventional traveling wave thermoacoustic driver 100 having a hot heat exchanger 130 and a cold heat exchanger 140, which are used to generate a temperature gradient across a regenerator 120. The conventional thermoacoustic driver 100 contains a compressible fluid that is capable of sustaining acoustic oscillations. To convert thermal energy into acoustic energy, acoustic traveling waves are introduced through the top of the conventional thermoacoustic driver 100. At substantially the same time, the cold heat exchanger 140 is cooled by passing an ambient temperature (or externally chilled) fluid 180 through pipe 160, and the hot heat exchanger 130 is heated by passing externally heated fluid 170 through pipe 150. The hot heat exchanger 130 and the cold heat exchanger 140 set up a temperature gradient in the regenerator 120, which is interposed between the hot heat exchanger 130 and the cold heat exchanger 140. The regenerator 120 comprises packing material that is fine enough so that the working fluid in the regenerator 120 is essentially in thermal equilibrium with the packing around it, but not so fine as to prevent the passage of acoustic waves through the regenerator 120.
Pressure oscillations produced by the acoustic traveling wave induce the compressible fluid in the regenerator to move down towards the hot end of the temperature gradient, or up towards the cold end of the temperature gradient. Consequently, when the compressible fluid moves down, the hotter regenerator packing heats and expands the compressible fluid; when the compressible fluid moves up, the colder regenerator packing cools and contracts the compressible fluid. As the acoustic traveling wave passes through the compressible fluid, it imparts time-dependent pressure and velocity oscillations to a small volume of the fluid at the wave's location. Since traveling waves are intrinsically phased such that the peak velocity and the peak pressure occur at substantially the same time, the processes undergone by the small volume of the fluid in the regenerator mimic the thermodynamic cycle of a Stirling engine. The thermodynamic cycle, therefore, results in conversion of thermal energy into mechanical energy. In other words, the traveling wave causes the compression, expansion, and fluid movement, which adds pressure and momentum to the waves, thereby amplifying the acoustic traveling wave as it passes through the regenerator.
As is known in the art, if the direction of the acoustic traveling wave is reversed from the hot heat exchanger 130 to cold heat exchanger 140, then the conventional thermoacoustic driver 100 may be used as a heat pump for refrigeration, air conditioning, or other cooling or heating applications. Since the operation of the conventional thermoacoustic driver 100 is known in the art, further discussion of the conventional thermoacoustic driver 100 is omitted here.
FIG. 2 is a diagram showing a cross-sectional view of a thermoacoustic Stirling heat engine (TASHE) 200 having a conventional thermoacoustic driver. As shown in FIG. 2, the TASHE 200 comprises a resonator 220, a variable acoustic load 210, and a thermoacoustic driving section 300. In one working example, the TASHE 200 is filled with helium at approximately thirty bars mean pressure. The use of high-pressure helium increases the acoustic power density of the TASHE 200, which permits acoustic effects to prevail over heat conduction losses.
FIG. 3 is a diagram showing, in greater detail, the thermoacoustic driving, section 300 of the TASHE 200 from FIG. 2. The thermoacoustic driving section 300 of the TASHE 200 comprises a toroidal acoustic feedback loop (or torus) 315 having a regenerator 330 interposed between a primary cold heat exchanger 325 and a hot heat exchanger 335. As described with reference to FIG. 1, the primary cold heat exchanger 325, the regenerator 330, and the hot heat exchanger 335 are configured to amplify acoustic traveling waves that propagate clockwise through the torus 315. At the junction 350, a portion of the amplified acoustic energy travels to the right towards the resonator 220 and the acoustic load 210, while the remainder is fed back, through the torus 315, to the cold end of the regenerator 330 to be amplified within the regenerator 330. Thus, when the acoustic traveling waves propagate clockwise through the torus 315, the thermoacoustic driving section 300 functions as a heat engine. Conversely, a counterclockwise propagation of acoustic traveling waves through the torus 315 attenuates the acoustic traveling waves, thereby resulting in a heat pump configuration in which heat is pumped from the cold heat exchanger 325 to the hot heat exchanger 335.
Additionally, the torus 315 contains an inertance section 305 and a compliance section 310. These sections 305, 310, along with the regenerator 330, define the properties of the acoustic waves in the thermoacoustic driving section 300. Each of these components 305, 310 and 330, are much shorter than an acoustic wavelength, though their specific geometries create the traveling wave acoustic phasing within the regenerator 330. They are also geometrically configured to reduce the acoustic velocity within the regenerator 330, thereby reducing viscous losses that would normally accompany the passage of an acoustic traveling wave through a conventional thermoacoustic driver 100, as shown in FIG. 1.
The thermoacoustic driving section 300 of the TASHE 200 further comprises a secondary cold heat exchanger 345, which, in conjunction with the hot heat exchanger 335, defines a thermal buffer tube 340. The thermal buffer tube 340 provides thermal isolation between the hot heat exchanger 335 and the rest of the TASHE 200 beyond the cold heat exchangers 325, 345.
One drawback of the TASHE 200 is that acoustic streaming in the thermoacoustic driving section 300 results in a convection current that travels clockwise around the torus 315, carrying thermal energy away from the regenerator 330 and out the secondary cold heat exchanger 345. Since this degrades the performance of the engine, it is desirable to eliminate or minimize any clockwise mean flow around the torus 315 and through the regenerator 330. As a result, the thermoacoustic driving section 300 of the TASHE 200 comprises a hydrodynamic mass-flux suppressor (or jet pump) 320 that is adjustable to minimize or eliminate any net flow of the compressible fluid around the torus 315. The operation of the mass-flux suppressor 320 relies on turbulence and the viscous dissipation of kinetic energy, so its use in suppressing the clockwise convection current is also accompanied by some dissipation of acoustic energy.
Also, in the TASHE 200, conduction of heat through the walls of the torus 315 can result in significant energy losses. These energy losses are due to heat conduction radially through the walls into the insulation or atmosphere surrounding the torus 315, and also due to axial heat conduction along the walls of the torus 315 between the hot heat exchanger 335 and the cold heat exchangers 325, 345, essentially bypassing the regenerator 330. For higher internal gas pressures as are typically present in the TASHE 200, greater wall thickness is required, which results in greater axial conduction losses. Additionally, crossflow heat exchangers 325, 335, 345, which are typically used due to geometric constraints, result in sub-optimal heat extraction and potentially enormous thermal stresses, especially in the hot heat exchanger 335.
Given these inefficiencies, a need exists in the industry for more efficient traveling wave thermoacoustic devices.